There are finite quantities of nonrenewable fossil fuels such as crude oil and natural gas that are currently being utilized to generate energy. Biomass (plant-derived material) is one the most important renewable energy resources. The conversion of biomass to fuels, chemicals, materials and power reduces the dependence on foreign oil and natural gas. Currently, biomass provides the only renewable alternative for liquid transportation fuel. Biomass use strengthens rural economies, decreases America's dependence on imported oil, reduces air and water pollution, and reduces greenhouse gas emissions.
A key challenge for promoting and sustaining the vitality and growth of the industrial sector is to develop efficient and environmentally benign technologies for generating fuels, such as hydrogen, from renewable resources. The generation of energy from renewable resources such as biomass, reduces the net rate of production of carbon dioxide, an important greenhouse gas that contributes to global warming. This is because the biomass itself consumes carbon dioxide during its life cycle.
Aqueous-Phase Reforming (APR) is a catalytic reforming process that generates hydrogen-rich fuel gas from oxygenated compounds derived from biomass (glycerol, sugars, sugar alcohols). The resulting fuel gas may be used as a fuel source for electricity generation via PEM fuel cells, solid-oxide fuel cells, internal combustion engines genset, or gas turbine genset. APR processes may generate light hydrocarbons (e.g. methane, ethane, propane, butane, propane, and hexane) and/or hydrogen by the reaction of oxygenated compounds with liquid water at low temperatures (e.g., less than 300° C.). The key breakthrough of the APR process is that the reforming can be done in the liquid phase. The APR process can occur at temperatures (e.g., 150° C. to 270° C.) where the water-gas shift reaction is favorable, making it possible to generate hydrogen with low amounts of CO in a single chemical reactor. Advantages of the APR process include: (i) performing the reaction at pressures (typically 15 to 50 bar) where the hydrogen-rich effluent can be effectively purified; (ii) generation of hydrogen-rich fuel gas at low temperatures without the need to volatilize water, which represents a major energy saving; (iii) operation at temperatures where the water-gas shift reaction is favorable, making it possible to generate high quality fuel gas with low amounts of CO in a single chemical reactor, (iv) operation at temperatures which minimize undesirable decomposition reactions typically encountered when carbohydrates are heated to elevated temperatures, and (v) utilization of agricultural derived feedstocks found in the United States.
The APR process takes advantage of the thermodynamic properties of oxygenated compounds containing a C:O stoichiometry of 1:1 to generate hydrogen from these oxygenated compound at relatively low temperatures in a single reaction step (see FIG. 1), in contrast to certain multi-reactor systems used for producing hydrogen via steam reforming of hydrocarbons. FIG. 1 was constructed from thermodynamic data obtained from Chemical Properties Handbook, C. L. Yaws, McGraw Hill, 1999.
Reaction conditions for producing hydrogen from hydrocarbons can be dictated by the thermodynamics for the steam reforming of the alkanes to form CO and H2 (reaction 1) and the water-gas shift reaction to form CO2 and H2 from CO (reaction 2).CnH2n+2+nH2O⇄nCO+(2n+1)H2  (1)CO+H2O⇄CO2+H2  (2)
FIG. 1 shows changes in the standard Gibbs free energy (ΔGo/RT) associated with reaction 1 for a series of alkanes (CH4, C2H6, C3H8, C6H14), normalized per mole of CO produced. The steam reforming of alkanes is thermodynamically favorable (i.e., negative values of ΔGo/RT) at temperatures higher than about 675 K. Oxygenated hydrocarbons having a C:O ratio of 1:1 produce CO and H2 according to reaction 3.CnH2yOn⇄nCO+yH2  (3)
Relevant oxygenated hydrocarbons having a C:O ratio of 1:1 include methanol (CH3OH), ethylene glycol (C2H4(OH)2), glycerol (C3H5(OH)3), and sorbitol (C6H8(OH)6). On FIG. 1, dotted lines show values of 1n(P) for the vapor pressures versus temperature of CH3(OH), C2H4(OH)2, C3H5(OH)3, and C6H8(OH)6 (pressure in units of atm). FIG. 1 shows that steam reforming of these oxygenated hydrocarbons to produce CO and H2 may be thermodynamically favorable at significantly lower temperatures than those required for alkanes with similar numbers of carbon atoms. Accordingly, the steam reforming of oxygenated hydrocarbons having a C:O ratio of 1:1 would offer a low-temperature route for the formation of CO and H2. FIG. 1 also shows that the value of ΔGo/RT for water-gas shift of CO to CO2 and H2 is more favorable at lower temperatures. Therefore, it is possible to produce H2 and CO2 from steam reforming of oxygenated compounds utilizing a single-step catalytic process, since the water-gas shift reaction is favorable at the same low temperatures where steam reforming of carbohydrates is possible.
While FIG. 1 shows that the conversion of oxygenated compounds in the presence of water to H2 and CO2 is highly favorable at this low temperature, the subsequent reaction of H2 and CO2 to form alkanes (CnH2n+2) and water is also highly favorable at low temperatures. For example, the equilibrium constant at 500 K for the conversion of CO2 and H2 to methane (reaction 4) is of the order of 1010 per mole of CO2.CO2+4H2⇄CH4+2H2O  (4)
The reforming reaction can be optimized not only to yield hydrogen, but to yield hydrocarbons. For example, the complete reforming of sorbitol yields 13 moles of hydrogen for every 6 moles of CO2 produced:C6H14O6+6H2O→6CO2+13H2  (5)
However, the more thermodynamically favored reaction consumes the hydrogen to yield a mixture of water and hydrocarbons:C6H14O6+xH2→aH2O+bCH4+cC2H6+dC3H8+eC4H10+fC5H12+gC6H14  (6)
Referring to FIG. 2, the individual reactions for the production of methane, ethane, and hexane are all thermodynamically favored (i.e., ΔGo/RT<0) across the entire temperature range presented in the graph. Moreover, production of these hydrocarbons is more favorable than the generation of hydrogen from the reaction of water with sorbitol. The thermodynamics for the formation of propane, butane, and pentane fit smoothly within the homologous series (between ethane and hexane), but these traces have been omitted from FIG. 2 for clarity. Thus, as described in full below, the present reaction can be optimized to yield a product mixture comprising almost exclusively hydrocarbons rather than hydrogen. FIG. 2 was constructed from thermodynamic data obtained from Chemical Properties Handbook, C. L. Yaws, McGraw Hill, 1999.
U.S. Pat. No. 6,699,457 to Cortright et al., as well as published U.S. patent application US 2005/0207971 A1 with Ser. No. 11/124,717 and filed May 9, 2005, which are incorporated herein by reference, disclose a method of producing hydrogen from oxygenated hydrocarbon reactants, including examples showing conversion of feedstocks comprising up to 10% glycerol, glucose, or sorbitol to hydrogen. The method can take place in the vapor phase or in the condensed liquid phase. The method can include the steps of reacting water and a water-soluble oxygenated hydrocarbon having at least two carbon atoms, in the presence of a metal-containing catalyst. The catalyst contains a metal selected from the group consisting of Group VIII transitional metals, alloys thereof, and mixtures thereof. The disclosed method can be run at lower temperatures than those used in the conventional steam reforming of alkanes.
U.S. Pat. No. 6,953,873 to Cortright et al., which is incorporated herein by reference, discloses a method of producing hydrocarbons from oxygenated hydrocarbon reactants, such as glycerol, glucose, or sorbitol. The method can take place in the vapor phase or in the condensed liquid phase (preferably in the condensed liquid phase). The method can include the steps of reacting water and a water-soluble oxygenated hydrocarbon having at least two carbon atoms, in the presence of a metal-containing catalyst. The catalyst can contain a metal selected from the group consisting of Group VIIIB transitional metals, alloys thereof, and mixtures thereof. These metals can be supported on supports that exhibit acidity or the reaction is conducted under liquid-phase conditions at acidic pHs. The disclosed method allows the production of hydrocarbon by the liquid-phase reaction of water with biomass-derived oxygenated compounds.
U.S. Pat. Nos. 6,964,757 and 6,964,758 to Cortright et al., which are incorporated herein by reference, disclose a method of producing hydrogen from oxygenated hydrocarbon reactants, such as methanol, glycerol, sugars (e.g. glucose and xylose), or sugar alcohols (e.g. sorbitol). The method can take place in the condensed liquid phase. The method can include the steps of reacting water and a water-soluble oxygenated hydrocarbon in the presence of a metal-containing catalyst. The catalyst contains a metal selected from the group consisting of Group VIIIB transitional metals, alloys thereof, and mixtures thereof. The disclosed method can be run at lower temperatures than those used in the conventional steam reforming of alkanes.
U.S. Pat. No. 4,223,001 to Novotny et al. discloses methods of generating hydrogen from an aqueous feedstock comprising a water-soluble alcohol, such as methanol or ethylene glycol, using a catalyst comprising a Group VIII metal, such as a homogeneous rhodium-containing catalyst (e.g., RhCl3.3H2O in the aqueous phase.
Cortright et al. describe the conversion of oxygenated compounds, methanol, ethylene glycol, glycerol, sorbitol, and glucose via aqueous phase reforming over a 3% Pt/Al2O3 catalyst. Reaction temperatures ranged from 498 to 538 K, system pressures ranged between 29 and 56 bar, and feed concentrations of 1 wt % oxygenated compound. Cortright, R. D.; Davda, R. R.; Dumesic J. A., Nature, Vol. 418, p. 964, 2002.
Davda et al. describe reaction kinetic studies of aqueous-phase reforming of 10 wt % ethylene glycol solutions over silica-supported metal catalysts. Reaction temperatures for this investigation were 483 and 498 K and reaction pressure of 22 bar. Results from this paper show that the overall catalytic activity of these catalyst decreases in the following order: Pt˜Ni>Ru>Rh˜Pd>Ir. Davda, R. R.; Shabaker J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A.; Appl. Cat. B: Environmental, Vol 43, p. 13, 2003.
Shabaker et al. describe reaction kinetic studies of aqueous-phase reforming of 10 wt % ethylene glycol solutions over Pt-black and Pt supported on TiO2, Al2O3, activated carbon, SiO2, SiO2—Al2O3, ZrO2, CeO2, and ZnO. Reaction temperatures were 483 and 498 K, and the reaction pressures were 22.4 and 29.3 bar, respectively. High activity for the production of H2 by aqueous-phase reforming was observed over Pt-black and over Pt supported on TiO2, carbon, and Al2O3; moderate catalytic activity for the production of hydrogen is demonstrated by Pt supported on SiO2—Al2O3 and ZrO2; and lower catalytic activity is exhibited by Pt supported on CeO2, ZnO, and SiO2. Pt supported on Al2O3, and to a lesser extent ZrO2, exhibits high selectivity for production of H2 and CO2 from aqueous-phase reforming of ethylene glycol. Shabaker, J. W.; Huber, G. W.; Davda, R. R.; Cortright, R. D.; Dumesic, J. A.; Catalysis Letters, Vol. 88, p. 1, 2003.
Davda et al. describe reaction conditions desired to generate hydrogen with low concentrations of CO via aqueous-phase reforming of ethylene glycol over a 3% Pt/Al2O3 catalyst. Reaction temperatures ranged from 498 K to 512 K, system pressure between 25.8 to 36.2 bar, and ethylene feed concentrations between 2 and 10 wt %. Davda, R. R; Dumesic J. A.; Angew. Chem. Int. Ed., Vol. 42, p. 4068, 2003.
Huber et al. describe the reaction of sorbitol to produce C1 through C6 alkanes over platinum-based catalyst with varying amounts of hydrogen added as a co-feed. In this paper, the platinum was loaded on either alumina or silica-alumina. This paper discussed the mechanism for this process through a bi-functional route involving acid-catalyzed dehydration reaction followed by a metal catalyzed hydrogenation reaction. Reaction temperatures were between 498 and 538 K, pressures between 25.8 to 60.7 bar, and feed concentrations of 5 wt % sorbitol. Huber, G. W.; Cortright, R. D.; Dumesic, J. A., Angew. Chem. Int. Ed., Vol. 43, p. 1549, 2004.
Davda et al. review aqueous-phase reforming of oxygenated compounds. Discussed are the effects of supports, supported metals, reaction conditions, and reactor configurations. Concentrations of oxygenated compounds were less than 10% in this paper. Davda, R. R.; Shabaker J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A.; Appl. Cat. B: Environmental, Vol 56, p. 171, 2005.
Huber et al. describe the effectiveness of tin modified nickel-based catalyst for the aqueous-phase reforming of oxygenated compounds such as ethylene glycol, glycerol, and sorbitol at 498 K and 538 K. Concentrations of oxygenated compounds studies in this investigation were less than 5 wt %. Huber, G. W.; Shabaker, J. W.; Dumesic, J. A.; Science, Vol. 300, p. 2075, 2003.
Previous patents and literature describe methods for the aqueous-phase reforming of water soluble oxygenated compounds at concentrations of 10 wt % or lower. Energy balances on the APR system indicate that significant energy losses can occur because of vaporization of water in the reactor system to maintain the partial pressure of water in the hydrogen gas bubbles formed in the reactor.
Thus, there exists a need for catalyst systems and processes that have higher activity levels to support high conversion of high concentrations of oxygenated hydrocarbon feedstocks in an aqueous reforming system.